The rotor system of a rotary aircraft, such as a helicopter, is the rotating part which generates lift of the aircraft. A helicopter's rotating wing assembly is normally called the main rotor, providing vertical lift of the helicopter. Traditionally, a main rotor consists of a mast, hub, and rotor blades, where the rotor blades are fastened on the hub by a tension-torsion strap assembly.
The usual way to provide a rotational force to the body of helicopter is to attach another set of rotating wings. These rotating wings are known as a tail rotor. The tail rotor produces thrust. By producing thrust in a sideways direction, the tail rotor keeps the body of the helicopter from spinning. Typically, the tail rotor consists of the same parts as those found in the main rotor.
While a rotary aircraft, such as a helicopter, is in flight the rotor blades tend to move radially outward due to centripetal force. The rotor blades are held in place against this outward motion by tension of the tension-torsion straps. Also, tension-torsion straps fastening the blades to the hub of the rotor system are articulated as the pitch of the aircraft is changed. A typical tension-torsion strap assembly consists of a spool having a central cylindrical spindle with a circular plate (or flange) at each end, with a strap wound around the spool. The articulation of the tension-torsion strap assembly creates a stress riser, both in tensile stresses and bearing stresses, at the edge of the spool when the wound material (i.e., the strap) is abruptly bent over the edge of the spool, resulting in potential damage of the strap.